backend+ui: search filter not working on Foreman Provider

Added empty constructor to the parameters, setting them using setters,
to solve some weird serialization issue with the parameters class.

Change-Id: I54110b5bda9a96b0b4a77256f6dbe2e07b1cb80b
Bug-Url: https://bugzilla.redhat.com/988012
Signed-off-by: Oved Ourfali <oourfali@redhat.com>